This invention relates in general to combined cycle power plants and, in particular, to an improved performance dual fuel combined cycle power plant capable of utilizing both distillate (liquid fuel) and natural gas fuels.
A combined cycle power plant utilizes a gas turbine and a steam turbine in combination to produce power, typically electric power. The power plant is arranged so that the gas turbine is thermally connected to the steam turbine through a heat recovery steam generator (HRSG). The HRSG is a noncontact heat exchanger which allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine exhaust gases. The HRSG is a large duct with tube bundles interposed therein whereby water is heated to steam as exhaust gases are passed through the duct. The primary efficiency of the combined cycle arrangement is, of course, due to the utilization of otherwise wasted gas turbine exhaust gases.
A key parameter in optimizing the combined cycle efficiency is that the highest efficiency is achieved at the lowest stack gas temperature measured at the outlet end of the HRSG. In a dual fuel combined cycle plant a limiting factor to achieving optimum efficiency is that a minimum tube surface temperature must be maintained in order to prevent the occurrence of sulfur cold end corrosion on the tube bundles. The inlet feedwater temperature affects the surface temperature of the turbine bundles, which must be maintained at a minimum temperature to prevent condensation of certain sulfur compounds produced by combustion of the liquid distillate fuels. The dew point of the corrosive sulfur compounds increases with increased concentration of sulfur in the fuel. No such limitation exists for gaseous fuels having negligible sulfur content.
The conventional method for optimizing a combined cycle plant efficiency is to design the HRSG and steam system to operate with an HRSG inlet feedwater temperature and a stack gas temperature that would prevent low temperature heat transfer surface corrosion commensurate with the highest level of sulfur content in the fuel expected to be burned in the specific application. If an alternate fuel such as natural gas is burned with lower fuel sulfur content, the HRSG stack gas temperature cannot be lowered to improve efficiency even though the sulfur compound concentration would allow it, since the HRSG inlet feedwater temperature is fixed. Conversely, if the HRSG were designed with inlet feedwater and stack gas temperatures commensurate with the lowest fuel sulfur content to be expected, the plant efficiency would be improved; however, the HRSG heat transfer surface would experience corrosion if fuel with a higher sulfur content were burned. This phenomenon is more fully explained in U.S. Pat. No. 4,354,347 assigned to the assignee of the present invention, issued Oct. 19, 1982 to Tomlinson and Cuscino and which is fully incorporated herein by reference.
The HRSG includes a plurality of interconnected tube bundles which may be identified from top to bottom (for the case of a vertical gas path) as an economizer, an evaporator and a superheater. The HRSG heat exchange process is a counterflow process in that the temperature of the hot exhaust gases decreases as they rise through the HRSG whereas the temperature of the steam water mixture in the tubes increases as it descends downwardly against the upward flow of hot exhaust gases.
It should be pointed out that dual fuel capability is a highly desirable attribute in power plant design since it will enable the operator to take advantage of fuel availability and cost factors. If maximum operational efficiency were not available in both modes then the attractiveness of dual fuel capability would be considerably lessened.